First principles study of hydrated/hydroxylated TiO nanolayers: From isolated sheets to stacks and tubes

Periodic density functional calculations are carried out to investigate the structure and the stability of hydrated/hydroxylated TiO ₂ layered compounds, nanosheets, and nanotubes. Due to a very efficient interlayer hydrogen bonding, the ABA-stacked "step 3" H ₂Ti ₃0 ₇ compound is found to be the most stable bulk phase, in agreement with the experiment. For single sheets in a water-rich environment other forms are instead favored, all close in energy, namely, "step 2" titanates, hydroxylized-anatase-like layers, and lepidocrocite-TiO ₂. Finally, it is shown that a lepidocrocite-TiO ₂ sheet, when hydroxylated only on one side, spontaneously forms a scroll-like nanotube. The nanotube diameter estimated from our models perfectly matches the ∼3 nm value observed for the internal diameters of Na-free titania nanotubes.